The present invention relates to measurement of float current, and more particularly to a float current probe and a measuring method that can be used for measuring float current for example in a battery string used as a backup power supply unit for telecommunication equipment, for battery state monitoring purposes.
Telecommunication systems are usually powered with direct current through rectifiers. Backup units are also provided since communication systems must remain operational for emergencies even during power failures, as prescribed by the national regulations in the majority of the industrialized countries. The backup units are usually formed of storage batteries, most of the time VRLA (Valve Regulated Lead Acid) batteries.
Real-time monitoring of the state and the condition of the batteries is necessary to detect eventual problems that would arise from a defective or an old battery. Many monitoring techniques have been developed for this purpose, like those involving impedance or mid-point measurements, yielding mixed results. Float current measurement has attracted attention just recently. There is a typical float current for every battery type, and this current tends to change as a function of the ageing and the temperature of the battery under scrutiny.
Float current is a precursory indicator of certain problems associated to battery backup units, as reported in the article entitled xe2x80x9cASSESSMENT OF LEAD-ACID BATTERY STATE OF CHARGE BY MONITORING FLOAT CHARGING CURRENTxe2x80x9d by Kyle D. Floyd et al., 1994 IEEE. A certain assessment of the batteries"" health can be carried out by time-trend analysis of the float current, thereby assisting in anticipating or detecting certain problems. By monitoring the float current, it is possible to detect a thermal runaway, which is some kind of race condition. This phenomenon is caused by a defective battery whose float current increases for any reason whatsoever. The direct effect of such an increase of the float current is an increase of the battery temperature which, in turns, causes an increase of the float current and so on, hence a race condition. The monitoring of the float current can also be used to detect an open string of batteries, since the float current then becomes nil.
One of the main desired features of a float current detector is the capacity to read very low DC current levels in the presence of high AC currents (up to 20 Amps) while being unaffected by high DC currents when a power failure occurs or during the battery recharging process. Some known float current detectors use a shunt with a high gain amplifier. The main drawback of this type of detector is that it requires the installation of a shunt, which is generally undesirable. A ferrite core with a Hall effect sensor can be used, but this kind of detector is sensible to the high remanence of this type of core.
Known in the art are U.S. Pat. Nos.: 3,657,639 (WILLIHNGANZ); 4,086,392 (MAO et al.); 4,266,189 (KARLIN et al.); 4,349,614 (WERTH); 4,707,795 (ALBER et al.); 4,886,980 (FERNANDES et al.); 4,935,688 (MISTRY et al.); 5,126,585 (BOYS: John T.); 5,665,966 (DAHL et al.); 5,786,682 (AIKEN et al.); 5,886,503 (McANDREWS et al.); 5,969,625 (RUSSO); and PCT patent application published under no. WO 98/53335 (JOKINEN), which show various devices and methods for monitoring string of batteries or for measuring currents, subjected to some or many of the following drawbacks: the battery branch must be opened to use the device; the device is not or is poorly immune to noise; in the case of a shunt device, the installation of the shunt is difficult and expensive; the measurement is inaccurate.
An object of the invention is to provide a float current probe and a measuring method, which overcome the drawbacks of the prior art devices and methods.
Another object of the invention is to provide such a float current probe and a measuring method, which provide more accurate results as it is less sensible to ambient temperature variations and construction details, and is much more immune to noise.
Another object of the invention is to provide such a float current probe and a measuring method, which can be installed and used without interrupting the operation and without opening a circuit branch of the equipment to be monitored.
Another object of the invention is to provide such a float current probe which may be defective without harming the equipment under monitoring.
According to the present invention, there is provided a float current probe for measuring a float current flowing in a conductor, comprising:
a split core adapted to be magnetically coupled to the conductor;
an excitation winding magnetically coupled to the core;
a field cancellation winding magnetically coupled to the core;
a switch circuit means coupled to the excitation winding, for controllably energizing and de-energizing the excitation winding in selectable forward and reverse polarity successively;
detecting means coupled to the excitation winding, for detecting when energy in the excitation winding reaches a predetermined threshold corresponding to a saturated core condition;
a timer responsive to the detecting means, and measuring first and second delays needed to saturate the core as a result of the excitation winding being energized through the switch circuit means in the forward and reverse polarity successively;
computing means for determining a magnetic field induced by the conductor based on the first and second delays measured by the timer; and
a current source means coupled to the field cancellation winding, for energizing the field cancellation winding with a current level and direction as a function of the magnetic field computed by the computing means, and thereby substantially cancelling the magnetic field induced by the conductor;
whereby the current level is representative of the float current in the conductor.
According to the present invention, there is also provided a method of measuring a float current flowing in a conductor, comprising the steps of:
magnetically coupling a split core with the conductor;
magnetically coupling an excitation winding and a field cancellation winding with the core;
cyclically energizing and de-energizing the excitation winding in forward and reverse polarity successively;
detecting when energy in the excitation winding reaches a predetermined threshold corresponding to a saturated core condition;
timing first and second delays needed to saturate the core as a result of the excitation winding being energized in the forward and reverse polarity successively;
determining a magnetic field induced by the conductor based on the first and second delays; and
energizing the field cancellation winding with a current level and direction as a function of the magnetic field, and thereby substantially cancelling the magnetic field induced by the conductor;
whereby the current level is representative of the float current in the conductor.
The probe can be used for measuring float current in any type of battery backup units subjected to a constant charging current for compensating intrinsic losses, like units based on VRLA or NiCd batteries. It can equally be used in UPS (Uninterruptible Power Supply) units involving AC backup power sources. The probe can be installed without opening the battery branch, as a result of the use of a split core. The measurement is more accurate as the core is always used at a virtually same magnetization point while the float current is measured as a function of the generated opposite magnetic field required to cancel the magnetic field induced by the conductor.